Can I ever email someone on a suppression list again?

It depends on why they're suppressed. The answer is different for unsubscribes, hard bounces, spam complaints, and manual suppressions.

Unsubscribes: You can only resume marketing contact if the person actively re-subscribes through a legitimate opt-in process. You can't decide to "give it another try" six months later or quietly remove them from your suppression list. If they want to come back, they need to sign themselves back up. For the mechanics of handling that, see re-subscribing after opting out.

Hard bounces: An address that hard bounced is typically non-deliverable. There's no compliance barrier to trying again (it's not a consent withdrawal), but operationally there's no point. you're just generating bounce events and hurting your sender score. The exception: if you have strong reason to believe the address has been corrected or reactivated, a single retry after a long gap is defensible.

Spam complaints: Technically you can email an address that complained if they later re-consent through a clean opt-in process. Practically, that person is at very high risk of complaining again. Most ESPs suppress complaint addresses permanently and won't allow re-importing them, which is the right call.

Manual or operational suppressions (legal disputes, competitors, known bad actors): these are your own business decision, and you can reverse them if the situation changes. Just update your records.

For all suppression types: never remove someone from a suppression list without a clear documented reason. The suppression is there to protect both of you.

See also legally required vs operational suppression for how the two categories work differently.